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The Briar Group, which owns ten restaurants, said, in a posting on its website, that the theft of credit card data "at our restaurants may have occurred from sometime in October 2013 to early November 2013."

The company owns the restaurants Anthem, City Bar, Brew Cafe, City Table, Gather Food and Drink, MJ O'Connor's, Ned Devine's, Solas, The Green Briar and The Harp. The Briar Group paid $110,000 in civil penalties in 2011 for a credit card breach that occurred in 2009.

It had been previously reported that a number of residents and visitors to the Boston area were victimized in mid-November of this year, but the Briar Group is among the first to detail the extent of the breach.

"Today we are reporting that the Briar Group's systems were indeed infiltrated," the statement said.

About 100 of the 13,000 people who attended the American Public Health Association convention in November have reported being victims. About 200 of the 8,000 people who attended the American Society of Human Genetics conference in October have reported unauthorized credit card purchases.

Attorney General Martha Coakley is investigating the breach, her office said in a statement.

"We have been looking into reports of compromised data by Seaport visitors and as part of our review, we urged the Briar Group to determine whether they were responsible for the breach in security of consumer credit card data,” spokesman Christopher Loh said in a statement. “Data breaches are a serious concern and we expect the Briar Group to assist consumers impacted by this breach.”

Victims' credit cards were used at bars, restaurants and other businesses in the Seaport area.

The Briar Group said not every customer at its restaurants may have been victimized, but urged its customers to monitor their credit card statements for fraud.

The company said it has been working closely with law enforcement agencies in an effort to identify the criminals behind the data breach.